Acoustic microscopes and other like acoustic inspection systems are utilized to determine physical characteristics of objects that cannot be ascertained by more conventional optical examination techniques; for example, see U.S. Pat. Nos. 3,585,848 and 3,790,281. Thus, acoustic microscopy can reveal details of mechanical structure, such as bonding, polymerization, elasticity, density, and viscosity that are not directly revealed by inspection of the electronic structure that is indirectly observed through optical inspection. In addition, an acoustic microscope, even in instances in which it shows the same structural detail as an optical microscope, frequently affords a major quantitative difference in its display as compared with an optical image. For example, the optical reflection from a water-air interface may amount to no more than two percent whereas the acoustic reflection is virtually total. Further, correlated acoustical and optical images of a given specimen may provide total information substantially exceeding that available from one of the images alone.
One problem encountered in acoustic microscopy is "acoustic speckle", comprising spurious image patterns created by interference between acoustic signals which pass through a specimen and arrive at a detection plane with a coherent time invariant phase relationship. This spurious detail may mask pertinent portions of the structure of the specimen and may afford a substantially erroneous image of it.
Another problem encountered in acoustic microscopes, and particularly in the microscope constructions described and illustrated in the aforementioned U.S. patents, pertains to the maintenance of effective alignment of the optical scanning portion of the microscope. Thus, it is frequently difficult to maintain the required precise alignment between the incident high intensity scanning beam that is used to develop acoustic information and the reflected scanning beam carrying that information, relative to the elements of the optical system that demodulate the reflected beam and convert it to usable electrical signals. These problems are accentuated by the frequent necessity for re-positioning the portion of the mechanism that supports the object under examination in order to compensate for differences in thickness and other characteristics of the objects examined.
One of the most practical and effective acoustic microscopes heretofore known, that of U.S. Pat. No. 3,790,281, utilizes light from the scanning beam that passes through the object under examination as a basis for development of an optical image that can be compared with the acoustic image. That system, however, does not afford a comparable or effective optical comparison image for specimens that are optically opaque. Nevertheless, for such optical specimens it is still desirable to have a direct optical image of the object under examination for comparison purposes in order to avoid erroneous conclusions that might be drawn from the acoustic image alone.